The field of conservation of artifacts has drawn considerable interest in the past few years and such conservation practices have been utilized to preserve archaeological materials. The chief concern in conserving these materials is to bulk up the cell wall structure of the waterlogged materials by introducing compounds that replace water in the damaged structure of the material. Allowing waterlogged wood to dry out without replacing the water with a bulking agent results in warping and excessive shrinkage, sometimes leading to total destruction of the wooden artifact.
Because the preserved wood needs to retain original coloration, have dimensional stability and integrity, one must be careful of the technique used to maintain these properties.
Prior art methods have used a variety of materials for preservation, namely, polyethylene glycol, acrylic resins and sugar.
For example, it is well-known in the field to use polyethylene glycols as conservation materials, especially in wood substrates. However, the materials so treated have a finite conservation time and one of the objectives of the instant invention is to prolong the conservation/preservation time of such substrates. For example, one of the inventors herein had acquired some wooden tongue depressors made out of birch wood, that had undergone controlled degradation and conservation using a variety of molecular weights of polyethylene glycol. It should be noted that polyethylene glycol has many physical forms and that the higher molecular weights are preferred for conservation. Such higher molecular weights are normally solid materials. However, after nearly ten years of preservation, the tongue depressors discussed supra showed signs of surface pooling and excessive flexibility that suggest that the polyethylene glycol was in a semi-liquid form and that the polyethylene glycol used to preserve the wood had de-stabilized, which eventually leads to instability of the preserved wood. In most cases, the coloration of the wood samples was translucent and unnatural in appearance. In some cases, the tongue depressors maintained a wet appearance as well as a general heightened degree of flexibility that exceeded the range of flexibility noted in the control samples.
Each of the above mentioned materials have their inherent benefits, but they also have their inherent disadvantages, such as in the case of the polyethylene glycol.
Because museums around the world have used a wide range of molecular weights of polyethylene glycol as bulking agents to conserve waterlogged and deteriorated wooden artifacts, and because polyethylene glycol treated artifacts require controlled curation, new means of stabilizing this bulking agent are necessary to ensure the long term survival of treated artifacts.
Further, some of these methods require a post heating step and high temperatures to complete the process and the wide variability in temperatures and high humidity can cause wood samples treated by polyethylene glycol and/or sugar to weep, resulting in pooled liquids on their surfaces such as that suggested supra. Thus, highly controlled storage of wood treated by these processes is essential for long term survival of the artifact.
Unfortunately, a great deal of wood recovered in the past from many archaeological sites was either ignored or discarded because conservation processes were either too costly or ineffective to guarantee results.
"Waterlogged" wood is employed herein as a reference to the wood whose water content is above its fibre saturation point, that is, when all of the sites available for hydrogen bonding are no longer available. Typically, this wood has undergone biological decay with chemical and physical changes inherent.
Under waterlogged conditions, water is in itself a conservation agent for the wood. It acts as the bulking agent, helping the wood to retain its shape and some degree of integrity. When waterlogged wood is allowed to dry, it suffers severe dimensional changes due to the collapse of the cell cavities and shrinkage of the cell walls.
Thus, waterlogged wood conservation has as its main objective, the avoidance of shrinking and deformation of the wooden artifacts without commensurate loss of aesthetics of the artifact.
Polyethylene glycol conservation of wood has been on-going since about the 1940's when the potential for treating waterlogged wood was first discovered. Polyethylene glycols are polymeric ethylene oxides that are currently commercially available from a number of manufacturers. They range in physical appearance from liquids to semi-solid waxes, to hard, wax-like solids. Typically, polyethylene glycol conservation is carried out in a batch process wherein the polyethylene glycol, having an average molecular weight of about 4000 is diluted with water and some sort of biocide. The wooden artifacts are immersed in this liquid, and heat is applied (generally less than 100.degree. C.) to evaporate water and facilitate penetration of the polyethylene glycol/biocide combination. It is known that the appropriate molecular weight of the polyethylene glycol to use is a function of the degree of deterioration of the wood being treated. Lower molecular weights are advised for relatively sound wood and higher molecular weights for wood badly deteriorated.
Some methods require that the wood have its initial bulking water displaced by using water soluble or water miscible solvents, followed by the use of the polyethylene glycol dilute solutions.